Purpose: :
The Delft Assessment Instrument for Strabismus in Young children (DAISY) aims to measure angles of strabismus fast and accurately in young children to improve outcome of surgery. In a previous study on healthy volunteering subjects with free head movement, an accuracy of 0.5 degrees was achieved. In this study, measurements of the angle of strabismus in various directions of gaze were done on children with strabismus. Whether the camera’s field of view (FOV) and depth of field (DOF) were sufficient to allow the children’s free head movement was of particular interest.

Methods: :
Eleven subjects with latent or manifest strabismus in the age of 3 to 15 were invited to fixate on red LED’s, positioned in all gaze directions (range: +25 to -25 degrees horizontally, -10 to +15 degrees vertically). Stereo images were obtained with two Prosilica GC2450 cameras. The angles of strabismus were derived using the coordinates of the pupil centers, fixation objects (LEDs) and corneal reflections created by four infrared light sources. Both the fixation LED’s and the cameras were at a distance of approximately 1150mm of the subject. The FOV was approximately 190x160mm. The DOF was 161mm. The room lights were on. Head rotation was assessed with a third camera and a face-tracking system (FaceAPI). Objective calibration of the projection of the fovea will be done with the automated Brückner test (#6363/D797 and #6373/D807, ARVO 2011), but was not yet operational for this measurement.

Results: :
The images of four subjects were out of focus. For one subject, at least one of the eyes was outside the FOV. Two subjects did not cooperate: one moved too much to be measured properly, the other seemed to be too tired to concentrate on the fixation LED’s. For four subjects, images were selected for data processing.

Conclusions: :
Measurements of 7 out of 11 subjects did not result in processable images, mainly because the subjects often moved out of the FOV and/or DOF. It was cumbersome to (re)direct the setup correctly on the patient. If precise translations of the setup becomes possible, it will be easier to keep the head within the FOV and DOF. Then, the current FOV and DOF are expected to be sufficient. Still, measurements of the remaining 4 subjects resulted in processable images. The results indicate the potential of DAISY to accurately determine the angle of strabismus in young patients.